In response to the demands of consumers who are driven both by ever-escalating fuel prices and the dire consequences of global warming, the automobile industry is slowly starting to embrace the need for ultra-low emission, high efficiency cars. While some within the industry are attempting to achieve these goals by engineering more efficient internal combustion engines, others are incorporating hybrid or all-electric drive trains into their vehicle line-ups. To meet consumer expectations, however, the automobile industry must not only achieve a greener drive train, but must do so while maintaining reasonable levels of performance, range, reliability, and cost.
The most common approach to achieving a low emission, high efficiency car is through the use of a hybrid drive train in which an internal combustion engine is combined with one or more electric motors. In general there are three types of hybrid drive trains: parallel hybrid, series-parallel hybrid, and series hybrid. In a parallel hybrid drive train, the power required to propel the vehicle may be provided by the internal combustion engine or the electric motor, either individually or together. In a series-parallel hybrid drive train, propulsive power is provided by both the internal combustion engine and the electric motor using a power splitter such as a planetary gear set. In a series hybrid drive train, propulsive power is only supplied by the electric motor and the internal combustion engine, which is coupled to a generator, is only used to charge the batteries as necessary.
While hybrid vehicles provide improved gas mileage and lower vehicle emissions, due to their inclusion of an internal combustion engine they still suffer from many of the inherent limitations of such a power source. For example, during operation the vehicle still emits harmful pollution, albeit at a reduced level compared to a conventional vehicle. Additionally, due to the inclusion of both an internal combustion engine and an electric motor(s) with its accompanying battery pack, the drive train of a hybrid vehicle is typically much more complex, resulting in increased cost and weight. Accordingly, several vehicle manufacturers are designing vehicles that only utilize an electric motor, or multiple electric motors, thereby eliminating one source of pollution while significantly reducing drive train complexity. The present invention provides an improved drive system for such a vehicle.